The present invention relates to an empty/load type brake control system for a railway freight car and, more particularly, to an empty/load changeover valve device that employs a proportioning valve to load-adjust the brake cylinder pressure in the empty range of car weight.
Single capacity brake equipment produces a brake shoe force that is independent of car loading, thus making it difficult to achieve desirably higher braking ratios for a loaded car without exceeding an empty car braking ratio sufficient to cause wheel slide. Sliding wheels are undesirable from the standpoint of reduced braking retardation, and slid-flat wheels.
Special brake equipment is therefore necessary to increase the loaded car braking ratio without incurring the consequence of a wheel slide condition when braking an empty car. Such equipment automatically adjusts brake shoe force according to the load condition of the car. These special equipments fall into two primary categories, dual capacity empty/load braking and multiple capacity or continuously variable braking.
In the dual capacity empty/load equipment, there are just two settings, one for "empty" braking and one for "load" braking, the changeover point between the "empty" and "load" settings being selected at some predetermined car weight, usually at 20% of the full load capacity weight. In arbitrarily selecting this changeover point, it will be appreciated that a given car can be generally under-braked by the reduced brake pressure when the car weight is in the upper end of the "empty" weight range, since essentially the same adhesion demand is available at the lower end of the "load" weight range at which maximum braking force is capable of being supported without sliding the car wheels.
In the variable load type equipment, braking pressure is proportioned according to the actual load, generally throughout the full range of car loading. It will be appreciated, however, that the proportioned brake pressure is selected in accordance with the maximum brake pressure (emergency) capable of being developed from the maximum running pressure normally carried by a train (110 psi). Therefore, when making relatively light service brake applications or when making a maximum brake application from a relatively low running pressure (70 psi), the proportioned brake pressure may be far less than that capable of being supported by the adhesion demand. Accordingly, less than optimum brake efficiency is realized under certain brake conditions with variable load type brake equipment, as well as single capacity equipment, in order to protect against wheel sliding on an empty car under maximum braking conditions.
In U.S. Pat. No. 5,005,915, an empty/load valve device is disclosed comprising a balance piston having equal opposing pressure areas, the balance piston being connected to a supply check valve via which brake pressure is connected from the car control valve device to the brake cylinder and released therefrom. One pressure area of the balance piston is subject to the upstream brake pressure under all conditions of car loading, while the opposing pressure area is either pressurized or de-pressurized depending upon the car being in an "empty" or "load" condition. In the "empty" condition, a differential pressure is created across the balance piston to establish a proportional mode of brake control in which the brake cylinder pressure is reduced by a certain percentage. A proportioning spring acts on the balance piston to vary this percentage of proportioning according to the actual car load during "empty" condition.
However, since an equalizing reservoir is required in such proportional type dual capacity equipments, to maintain proper equalization pressure at the car control valve and thereby prevent an over-reduction of brake pipe pressure from producing brake cylinder pressure in excess of the desired empty brake cylinder pressure, it is possible under certain operating situations to experience an undesirable drop in brake cylinder pressure during changeover from "load" to "empty" condition of brake control. For example, should the buildup of upstream brake cylinder pressure supplied by the car control valve be terminated when changeover between "empty" and "load" occurs, the connection of upstream brake pressure to the previously vented equalizing reservoir could create such a drop in the upstream pressure that the pressure differential across the balance piston would no longer be able to hold the supply check valve closed against the proportioning spring. The sufficiency of this reduction in the upstream brake pressure could arise, for example, where the pipe volume between the car control valve and empty/load changeover valve is relatively small as compared to the volume of the equalizing reservoir. Consequent opening of the supply check valve by the proportioning spring, in turn, connects the brake cylinder pressure to the equalizing reservoir, resulting in the aforementioned undesirable drop in brake cylinder pressure.